SUB-OKDKK 1! 



HOLOCHO AN IT ES M I XOCHO AN ITES 



515 



and taper off at the centre into a spin- that is .-oim-tiinrs tubular and 1 

 tlattrurd and elliptical. This is tin- .,//...//,/,/'//?. 

 Ordovician and Silurian. 



Narthecoceras, Hyatt. Long, cylindrical, stafl'-likc 

 orthoceracones. Siphuncle large and filled with organic 

 deposits having a radiating fibrous structure like the 

 guard of a Belemnite. Endocones and an endosiphuncle 

 developed. Septa continuous around the siplnmrlr. 

 Ordovician. 



Nanno, Clarke. Similar to the preceding, hut endo- 

 siphuncle present only at the apical end. Siphuncle 

 close to the shell, so that sutures appear to l>rn<l 

 apically into a lobe passing around the siphuncle. 

 Trenton Limestone. 



Family 2. Piloceratidae. 



Shorter and stouter orthoceracones and cyrtoceracones 

 with relatively larger siphuncles than in Endoceratidae, 

 and more variable in their internal deposits. Septa are 

 more concave and sutures more sinuous. Camerae empty 

 and funnels similar. 



Piloceras, Salter. Breviconic cyrtoceracones with 

 very large siphuncle and well - defined endocones. 

 Ordovician. 



iollo\v, or 



FIG. 1056. 



Endoceras protfiforme, Hall, onlu- 

 vician ; New York. Longitudinal 

 section showing funnels and endo- 



Family 3. Cyrtendoceratidae. 



Gyroceracones and nautilicones having large siphuncles filled with organic deposits or 

 empty, but with endocones obscure or absent, and no endosiphuncles. 



Cyrtendoceras, Kemele. Gyroceracones with siphuncle near the dorsum and lillrd 

 with calcareous deposits. Ordovician. 



Sub-Order B. MIXOCHOANITES. Hyatt. 



Orthoceracones and cyrtoceracones having expanded living chambers with contracted 

 apertures in the gerontic stage of specialised genera. The oldest septa are bent ,s7/"/-y/// 

 orad, forming a series of dorsal saddles, and the siphuncle becomes highly modified. 

 Primitive genera have the septa deeply concave or approximately sub-conical, the siphuncle 

 small and empty, and the septa sometimes more or less imperfect on the ventral side in the 

 gerontic stage. Specialised forms have siphuncles with short, straight funnels in flic 

 young, and in the ephebic stage collars are built around the oral openings, thus becoming 

 parallel to some forms of Goniatitidae that have similar composite funnels. 



Family 1. Ascoceratidae. 



Cyrtoceracones, smooth or annulated. Siphuncle with long funnels only in the young 

 and later stages of primitive genera, but cottars are added in later stages of specialised 

 forms, and segments become nummuloidal in gerontic stage. Septa often more or less 

 imperfect around siphuncle and on the ventral side. 



Choanoceras, Lindstr. Sections depressed elliptical. Gerontic stages have no 

 saddles, and living chamber uncontracted. Ordovieian and Silurian. 



